The handler.has() method is a trap for the [[HasProperty]] object internal method, which is used by operations such as the in operator.
const handler = {
has(target, key) {
if (key[0] === "_") {
return false;
}
return key in target;
},
};
const monster = {
_secret: "easily scared",
eyeCount: 4,
};
const proxy = new Proxy(monster, handler);
console.log("eyeCount" in proxy);
// Expected output: true
console.log("_secret" in proxy);
// Expected output: false
console.log("_secret" in monster);
// Expected output: truenew Proxy(target, {
has(target, property) {
}
})The following parameters are passed to has() method. this is bound to the handler.
The has() method must return a Boolean indicating whether or not the property exists. Other values are coerced to booleans.
This trap can intercept these operations:
in operator: foo in proxywith check: with(proxy) { (foo); }Reflect.has()Or any other operation that invokes the [[HasProperty]] internal method.
The proxy's [[HasProperty]] internal method throws a TypeError if the handler definition violates one of the following invariants:
Reflect.getOwnPropertyDescriptor() returns configurable: false for the property on target, the trap must return true.Reflect.isExtensible() returns false on target, and Reflect.getOwnPropertyDescriptor() returns a property descriptor for the property on target, the trap must return true.The following code traps the in operator.
const p = new Proxy(
{},
{
has(target, prop) {
console.log(`called: ${prop}`);
return true;
},
},
);
console.log("a" in p);
// "called: a"
// trueThe following code violates an invariant.
const obj = { a: 10 };
Object.preventExtensions(obj);
const p = new Proxy(obj, {
has(target, prop) {
return false;
},
});
"a" in p; // TypeError is thrown